1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly to a technique of separating and feeding sheets by blowing air to the sheets.
2. Description of the Related Art
In the related art, an image forming apparatus, such as a printer or a copying machine, is equipped with a sheet feeding device which feeds sheets one by one from a sheet storage portion in which a plurality of sheets is stored. As this kind of sheet feeding device, there is an air sheet feeding system which blows air to the end of a sheet bundle stored in the sheet storage portion to blow up a plurality of sheets and causes only one sheet to be adsorbed onto an adsorption conveyance belt disposed above the sheet, thereby feeding the sheet (see Japanese Patent Laid-Open (JP-A) No. 7-196187).
In a sheet feeding device of this air sheet feeding system, a sheet tray, which stacks sheets in a sheet storage portion where multiple sheets are stored, is configured to be able to be lifted and lowered. In addition, on top of a sheet storage case are provided an adsorption conveying portion and an air blowing portion. The adsorption conveying portion adsorbs and conveys a sheet and the air blowing portion blows air to the end of a sheet bundle on the sheet tray to blow upward a plurality of sheets in order to separate the sheets one by one.
Further, in the case of feeding sheets by adsorption, the sheets are conveyed in such a manner that an air blowing portion blows air to an upper portion of a sheet bundle which is stacked on a sheet tray to blow upward a plurality of sheets and an adsorption conveying portion adsorbs and conveys the top sheet out of the blown-upward sheets. Here, this kind of air sheet feeding system is adopted in a highly productive apparatus which can feed 70 sheets or more of A4 size or Letter size per minute. In this apparatus, the sheet tray is usually equipped with a lifting mechanism which is moved in a vertical direction while keeping its posture horizontal.
In addition, this kind of sheet feeding device is also equipped with a sheet surface detecting mechanism which controls the position of the top surface of a sheet bundle which is stored in the sheet storage case. This sheet surface detecting mechanism is equipped with a sheet surface detecting member which is installed to be able to be lifted and lowered while keeping its parallel (horizontal) posture, and is lifted up by being pushed up by the top sheet blown upward due to the air which is blown by the air blowing portion (see U.S. Pat. No. 7,744,081 B2). Accordingly, the height of the top surface of the sheet bundle can be appropriately controlled by detecting a change in the output of a sensor in response to an upward motion of the sheet surface detecting member, and thus the sheet bundle can reliably be separated when a sheet is adsorbed onto the adsorption conveyance belt.
Furthermore, this sheet surface detecting member usually extends downward from between the adsorption conveyance belts, but when the sheet is adsorbed onto the adsorption conveyance belt, the sheet surface detecting member retracts into an adsorption duct by being pushed by the sheet, so as not to hinder the sheet from being adsorbed onto the adsorption conveyance belt.
On the other hand, sometimes the sheet which has a weak rigidity and is thin to have a basic weight of about 50 g/m2 is adsorbed and fed in this kind of a related art sheet feeding device. In this case, when the sheet is adsorbed onto the adsorption conveyance belt, as illustrated in FIG. 9A, the sheet S deflects upward, and thus ends up entering into the adsorption duct 51 through a storage hole 51H1, provided for storage of the sheet surface detecting member 161, formed at the bottom of the adsorption duct 51 along with the sheet surface detecting member 61.
In this case, when the adsorption conveyance belt 21 rotates as it is, an upwardly protruding curve-shaped deflection Q1 of the sheet S which has entered into the storage hole 51H1 gets out of the storage hole 51H1. However, when it gets out of the storage hole 51H1, it then reaches the point where the end of the sheet in the sheet conveying direction deflects downward due to the rigidity of sheet S. When the sheet deflects downward in this way, as illustrated in FIG. 9B, this deflection Q2 may be caught by a guide plate 35 which guides the sheet or by the end of an air blowing nozzle (not illustrated) or the like, or may be strongly pressed against the surface of the guide plate, so that a paper jam occurred in many cases.
This invention is made in view of the above circumstances, and thus the invention provides a sheet feeding device and an image forming apparatus which are able to feed a sheet while preventing a sheet from being caught even when using a thin sheet.